Printed circuit boards are formed from a layer of conductive material (commonly, copper or copper plated with solder or gold) carried on a substrate of insulating material (commonly glass-fiber-reinforced epoxy resin). A printed circuit board having two conductive surfaces positioned on opposite sides of a single insulating layer is known as a "double-sided circuit board." To accommodate even more circuits on a single board, several copper layers are sandwiched between boards or other layers of insulating material to produce a multilayer circuit board.
To make electrical connections between two or more circuit layers on opposite sides of a double-sided circuit board, a hole is first drilled through the two conducting circuit layers and the insulator board. These holes are known in the art as "through holes," particularly if they extend through the entire circuit board. Through holes are typically from about 0.05 mm to about 5 mm in diameter and from about 0.025 mm to about 6 mm long. The through hole initially has a nonconductive cylindrical bore communicating between the two conductive surfaces. A conductive material or element is positioned in the through hole and electrically connected with the conducting sheets or layers to complete an electrical connection.
Like double-sided circuit boards, multilayer circuit boards also use holes in an intervening insulating layer to complete circuits between the circuit patterns on opposite sides of the insulating layer, as well as intermediate layers. Unless the context indicates otherwise, references in this specification to "through holes" refer to these holes in multilayer boards as well, even if they do not literally go through the entire circuit board.
When a through hole is drilled through a double-sided or multilayer circuit board, the act of drilling leaves a smear of insulating material in the barrel of the hole, as well as on the conductive surface. This smear must be removed prior to positioning or depositing the conductive material or element in the through hole if conductive contact between the through hole and the conducting sheets or layers is to be achieved.
Smear removal processes and solvents are known in the art. Several mechanical and chemical desmear methods that are known in the art are described in U.S. Pat. No. 4,601,783, issued Jul. 22, 1986 to Krulik. One common smear removal process utilizes an alkaline permanganate solution to remove the resin smear. U.S. Pat. No. 4,820,548 to Courduvelis et al. describes an alkaline permanganate desmear process that involves three chemical steps. In the first step, called solvent etch, a solvent is applied to attack and soften the resin structure of the smeared material; in the second step, a permanganate oxidizer is applied to remove the swelled resin; in the third step, a neutralizer is applied to neutralize and remove the permanganate from the resin surface. Suggested solvents for use in the disclosed desmear process include alkaline solutions of propylene glycol ethers. Other known solvents for use in permanganate desmear processes include such chemicals as ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, or tripropylene glycol methyl ether. Permanganate oxidizers used in the desmear process include alkaline solutions of sodium, potassium or lithium permanganate. Neutralizers used in the permanganate desmear process are typically oxalic acid or aqueous acidic solutions such as dilute sulfuric acid or hydrochloric acid. Other known neutralizers are acidified stannous chloride, hydroxylamine hydrochloride or formaldehyde.
The known solvents used in permanganate desmear processes work fairly well in removing resin smear from a laminate material extensively used and known in the industry by the designation FR4, which is a fire retardant epoxy resin fiberglass cloth laminate. In recent years, however, specialized materials, resins and fillers have been used to develop new printed circuit board laminates having improved properties adequate for both state-of-the-art processing and performance requirements. Due to their chemical structure, the specialized resins developed for high performance laminates are very difficult to desmear using permanganate chemical processes. One such specialized resin is an epoxy, polyimide, cyanate ester resin system marketed by Allied Signal under the trademark RCC.RTM.. Another high performance resin is BT epoxy, a combination of bis-maleimide triazine (BT) resin and an epoxy resin. Permanganate desmear processes can be used for smear removal of such high performance or specialized resins, but satisfactory smear removal can only be achieved at the highest recommended levels of solvent and permanganate. For many users, such high levels are not practical.